Power semiconductor devices, such as those fabricated from SiC (silicon carbide), may be designed to operate at very high operating temperatures (e.g., greater than 250° C.). Such power semiconductor devices may be bonded to a cooling device, such as a heat sink or a liquid cooling assembly, for example. The cooling device removes heat from the power semiconductor device to ensure that it operates at a temperature that is below its maximum operating temperature. The bonding layer that bonds the power semiconductor device to the cooling device must be able to withstand the high operating temperatures of the power semiconductor device.
Transient liquid phase (TLP) or diffusion bonding or soldering are methods of high temperature bonding that may be applied. For example, TLP bonding results in a bond layer having a high temperature melting point. A typical TLP bond consists of two different material compounds: a metallic layer and an intermetallic layer or alloy. Generally, the intermetallic layer is formed during an initial melting phase wherein a low melting temperature material, such as tin, diffuses into high melting temperature materials, such as copper or nickel. Although the intermetallic alloy has a high re-melting temperature, conventional processes use tin along with paste including metal particles such as copper or nickel and apply a direct, one-step heating. The assembly is heated to a temperature that is, for example, a low melting point of tin. However, with such one-step heating, the paste impedes the path of the formed tin solder and prevents the solder from more fully coating other metal particles, which may result in a weaker bond layer.
Accordingly, a need exists for alternative methods for high temperature bonding of substrates for forming a strengthened bonding layer between a pair of substrates.